JPS6230849A - Directly quenched and tempered steel having superior sscc resistance characteristic - Google Patents

Abstract

PURPOSE:To obtain a steel having superior SSCC resistance characteristic by direct quenching and tempering by controlling the structure of a low-alloy steel by a direct quenching process. CONSTITUTION:The composition of a steel is composed of, by weight, 0.15-0.35% C, 0.01-0.5% Si, 0.4-2% Mn, 0.001-0.1% acid-sol. Al and the balance Fe with inevitable impurities. The steel is hot worked at the Ac3 transformation point or above, directly quenched and tempered at the Ac1 transformation point or below to form a tempered martensite structure contg. old austenite grains drawn in >=2 ratio R represented by a formula R=dL/dZ (where dL is the grain size of old austenite in the rolling direction and dZ is the grain size of old austenite in the thickness direction).

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Application of Mado) The present invention relates to a steel with excellent sulfide stress corrosion cracking resistance (hereinafter referred to as 5 SCC resistance) obtained by direct quenching and tempering.

(Prior art and its problems) In recent years, many gas fields and oil fields for sour gas and sour oil have been developed, and along with this, there is a demand for steel materials for oil country tubular goods having excellent resistance to 88 CC%.

Conventionally, C-8i-Fdn steel or C-8i-Or-
Although Mo steel is manufactured by quenching and tempering, these steel materials still have the following problems.

■C-8i-tane steel has a yield strength of 60 to 80KIF/-
Sufficient SSCC resistance properties have not been obtained in high-strength steels of this grade.

■C-S i -Mn -Cr -Mo steel is C-8i
Although a higher 5scc resistance than -1'vln steel is obtained, since the amounts of added Cr and Mo are large, it is inevitable that the manufacturing cost will be high.

■Corrosive environmental conditions in sour gas and sour oil are becoming increasingly severe, and C-8i -Mn -Cr
-Mo steel is also required to have higher SSCC resistance, but this cannot be satisfactorily met.

(Means for Solving the Problems) The present invention was devised through repeated research in view of the above-mentioned circumstances, and its purpose is to achieve microstructural control by a direct quenching process in the range of low-alloy steels. Utilizes it to maintain the same strength level as before, yet with a superior 5sc resistance.
The objective is to provide steel with c characteristics.

In order to achieve this object, the present invention focuses on and defines the prior austenite grain elongation of a tempered martensitic structure obtained by directly quenching and tempering steel with a predetermined composition after hot working, that is, C: 0.15~0.35w
t%, S plate = 0.01-0.50 wt%, Mn:
0.4-2.0 wt%, 5otAL: 0.
001 to 0.10 wt%, the balance being Fe and unavoidable impurities, and if necessary 1 wt%
% or less Cu% 1wt% or less Ni, 2wt% or less C
r, 2wt% or less Mo, 0.1wt or less (D Nb
, 0.2wt % or less CD V , 0.1wt
% or less Ti, 0.1wt or less REM, 0.1
Ac refers to steel to which one or more types of Ca are added in an amount of wt % or less.

Directly quenched after hot working above the transformation point, then Act
In the tempered martensitic structure obtained by tempering below the 5i state, the prior austenite grain elongation is expressed by: = (prior austenite grain size dt in the rolling direction)/(prior austenite grain size aZ in the wall thickness direction) It is characterized by having a degree of 2 or more.

The present invention will be explained in detail below.

First, the present inventors focused on the shape of crystal grains (prior austenite grains in the case of low alloy steel that has undergone quenching and tempering treatment) as one of the metallurgical factors that improves the 5 SCC resistance. Then, in order to examine the influence of the shape of prior austenite grains on the 5SCC resistance, we investigated the SSCC resistance of steel materials subjected to direct quenching and tempering treatment, in which the shape of prior austenite grains can be controlled by changing the hot working conditions. .

No. 11 is 0.2IC-0,228i-1,32Mn steel (
YS=74 to 76K17wm"), the rolling reduction ratio in the non-recrystallization temperature range on the 5 SCC resistance property when directly quenched 15 seconds after finishing rolling is shown.

As is clear from this first factor, under direct quenching and tempering conditions, if the rolling reduction in the non-recrystallized temperature region is 30 inches or more, the elongation of the prior austenite grain size [R (prior austenite grain size in the rolling direction dL/ The prior austenite grain size dz in the wall thickness direction is 2 or more, and in this region SS resistance
The CC characteristics (in this case, the critical cracking stress Sc value in a three-point bending test in a 0.5% acetic acid H, S saturated solution) are improved.

On the other hand, when the reduction rate in the non-recrystallized region is set to 30 or less, the degree of elongation of the prior austenite grain size becomes 2 or less, and the improvement in the SC value is not so large.

The present invention is based on this knowledge, and the reason why the 5scc I￥f resistance is improved by elongation of prior austenite grains is as follows.
This is due to the fact that the propagation of cracks generated on the steel surface in the thickness direction is easily deflected along the prior austenite grain boundaries extended in the rolling direction, and the prior austenite elongation rate R is 2.
If it is less than that, it is not effective in improving the SSCC resistance characteristics.

Therefore, the elongation of prior austenite grains, which is effective in improving the 5 SCC resistance, is promoted by the addition of a small amount of Nb or Ti. Figure 2 shows 0.2C-0,2Si-1,3Mn-
This shows the results of examining the effect of Nb addition on Nb steel (YS=74-76Si5), and the SSCC resistance properties are as follows:
Even under the same hot rolling-direct quenching and tempering conditions, the addition of a small amount of Nb significantly improved the results.

In other words, for Nb-free steel, the elongation R) is 2 only under rolling conditions (1) (70% rolling reduction at 900°C or lower, finishing rolling temperature 870°C); , the rolling finish temperature is 920°C), the value becomes <2.

On the other hand, in Nb@processed steel, both rolled strips f'F have R>
2, and it can be seen that the 5scc resistance characteristics are all significantly improved under such conditions.

5sc resistance due to elongation of such prior austonite grains
The effect of improving the c characteristic is as shown in the examples described later.
-Si -Mn -Cr-kio steel (e.g. 0.3
C-0,2S i -0,5Mn-I Cr -0,5
Of course, it can also be obtained with Mo steel).

In view of the above points, the present invention adopts the configuration described above, and the reasons for the limitations will be described below.

C must be 0.0 to obtain a completely hardened structure by direct quenching.
This is because 15 Wt 4 or more is required, and in order to prevent quench cracking by direct quenching, it needs to be 0.35 wt % or less.

8i is 0.01wt to completely deoxidize molten steel.
4 or more is necessary, but the content of 0.5wt4 or more is because it impairs the ductility and toughness of the steel material.

Mn needs to be at least 0.4 wt % from the viewpoint of ensuring the hardenability of the steel material, but it is specified because the content of 2 wt % or more impairs the ductility and 5 SCC resistance of the steel material.

5otAt is 0.00 in order to completely deoxidize molten steel.
Although 1 wt% or more is required, the content of 0.1 wt tip or more impairs the ductility and toughness of the steel material.

As mentioned above, Nb and Ti are effective elements for promoting elongation of prior austenite grains, but if added in excess of 0.1 wt%, the effect becomes saturated, so an upper limit was specified.

Cu, Ni, Cr, and Mo can be added in amounts below their respective limits to improve hardenability without impairing the 88CC resistance, and to increase material strength through solid solution strengthening and precipitation strengthening. Addition in excess of this amount is undesirable from the viewpoint of manufacturing costs.

By adding 0.2wt or less, ■ increases material strength through precipitation strengthening without impairing 5scc resistance properties,
．． Addition of 2 wt% or more is undesirable from the viewpoint of manufacturing cost.

REM, Ca makes the morphology of Mn8 inclusions spherical and improves the ductility of the steel material when added below the limited value, but when added above the limited value, it actually impairs the ductile toughness and also reduces the 5scc resistance.
This is because it does not affect the characteristics.

The present invention controls the structure of steel with the above-mentioned limited composition by direct quenching process, and in the tempered martensitic structure, prior austenite grain elongation g R = dL/dz
) 2. The reason for this has already been mentioned, but it should be added that the elongated prior austenite grains obtained by hot mechanical processing in the non-recrystallized region are preserved only by direct quenching, and cannot be recycled. Expanded prior austenite grains cannot be obtained by heating and quenching.

(Example) Next, specific examples of the present invention will be shown together with comparative examples.

First, the chemical fractions of the inventive steel and comparative steel are calculated as follows:
Table 2 shows the hot rolling and heat treatment conditions of each steel shown in Table 1, as well as the results of examination of the degree of elongation of prior austenite grains, yield strength, and 5 SCC resistance properties.

From Tables 1 and 2 above, in non-inventive steel, Nb
Regardless of the presence or absence of addition, the prior austenite grain expansion elongation is 2 or more, but the prior austenite grain expansion elongation of all comparative steels is 2 or less. All of the steels of the present invention have approximately the same strength as the corresponding comparative steels, and have excellent 5scc resistance characteristics.

(Effects of the Invention) According to the present invention as described above, it is possible to obtain a steel that maintains the required strength level and has excellent 5SCC resistance, especially in the range of low alloy steel. Therefore, it is possible to provide a steel material suitable as a steel material for oil country tubular goods in gas fields, oil fields, etc. for sour gas and sour oil.

[Brief explanation of drawings]

FIG. 1 is a graph showing the influence of the rolling reduction in the non-recrystallization temperature range on the 5 SCC resistance characteristics, and FIG. 2 is a graph showing the influence of Nb addition on the 5 SCC resistance characteristics.

Claims (1)

[Claims] C: 0.15-0.35 wt%, Si: 0.01-0.
50wt%, Mn: 0.4-2.0wt%, SolAl
: 0.001 to 0.10 wt%, balance Fe and unavoidable impurities basic composition, and if necessary, 1 wt% or less Cu, 1 wt% or less Ni, 2 wt% or less Cr, 2 wt% or less Mo, N less than 0.1wt%
b, V of 0.2 wt% or less, Ti of 0.1 wt% or less,
Steel to which 0.1 wt% or less of REM and 0.1 wt% or less of Ca are added is directly quenched after hot working at a temperature above the Ac_3 transformation point, and then tempered at a temperature below the Ac_1 transformation point. In the tempered martensite composition thus obtained, the prior austenite grain expansion elongation represented by R = (prior austenite grain size d_L in the rolling direction)/(prior austenite grain size d_Z in the wall thickness direction) is 2 or more. A steel with excellent SSCC resistance due to direct quenching and tempering.